The invention relates generally to a fiber Bragg grating based temperature sensing device and, more particularly, to thermally stabilized fiber Bragg grating based temperature sensing devices that can be operated at elevated temperatures beyond 1000 K.
Temperature sensing is essential for a safe and efficient operation and control of many industrial processes. Industrial processes such as coal boiler operation, combustion, power generation, and gasification involve the measurement of high temperatures either for real-time industrial process monitoring or for control and optimization.
In general, there are several techniques used for measurement of temperatures beyond 1000 K. Some of the commonly used techniques include a thermocouple, pyrometry and blackbody measurement. Further, Fiber Bragg grating (FBG) based fiberoptic temperature sensors have been found to be a potential method for elevated temperature measurement. FBG is a high quality reflector constructed in an optical fiber that reflects particular wavelengths of light and transmits other wavelengths. This is generally achieved by adding a periodic variation to a refractive index of the fiber. It is advantageous to use FBG for power generation industrial process monitoring because of low mass, high sensitivity, multiplexing, multi-point distribution, and electromagnetic interference immunity.
However, the ultraviolet (UV) light induced FBG sensors exhibit undesirable thermal instability at elevated temperatures. The grating is generally completely erased at a temperature of around 900 K after only a few, for example 2-4, hours of operation. Conventional UV inscribed FBG commonly includes a Type-I, a Type-IIA and a Type-II grating. The Type I grating is a periodic refractive index modulated grating structure that degrades at temperatures higher than about 500 K, again, after only a few, for example 2-4, hours of operation. As used herein, the term ‘erasure’ refers to a change in refractive index of the grating. A second type of grating that is inscribed after a Type-I grating and is also referred to as Type-IIA grating, is a negative refractive index grating and is generally found to have an erasure temperature of about 800 K after only a few, for example 2-4, hours of operation. A Type-II grating inscribed after a Type-IIA grating has a broad reflective spectrum that is undesirable for high temperature sensing applications.
Therefore, a need exists for an improved engineering fiber material and a grating inscription method to obtain highly thermal stabilized temperature-sensing device that addresses one or more of the problems set forth above and survives harsh environment conditions.